This invention relates to a magnetic disk drive such as a flexible or floppy disk drive for reading/writing data from/to a magnetic disk medium of a flexible or floppy disk and, more particularly, to a position detection unit for use in positioning of a carriage for supporting a magnetic head.
As is well known in the art, a flexible or floppy disk drive (which is abbreviated to "FDD") of the type described is a magnetic disk drive for carrying out recording/reproducing of data for a magnetic disk medium of a flexible or floppy disk (which is abbreviated to "FD") inserted thereinto. In recent years, advances have been made to make the storage capacity of FDs larger. As a result, FDs having the storage capacity of 128 Mbytes (which are called large capacity FDs) have been developed as compared with FDs having storage capacity of 1 Mbyte or 2 Mbytes (which are called small capacity FDs). Under the circumstances, FDDs which are capable of recording/reproducing data for magnetic disk media of the large capacity FDs have also been developed.
Hereinunder, the FDDs capable of recording/reproducing data for magnetic disk media of the large capacity FDs alone are called high density exclusive type FDDs. The FDDs capable of recording/reproducing data for magnetic disk media of the small capacity FDs alone are called low density exclusive type FDDs. Furthermore, the FDDs capable of recording/reproducing data for magnetic disk media of both of the large and the small capacity FDs are called high/low density compatible type FDDs. In addition, the high density exclusive type FDDs and the high/low density compatible type FDDs are collectively called high density type FDDs.
A main difference in the mechanism between the low density exclusive type FDD and the high density type FDD is the structure of a driving arrangement for moving a carriage supporting a magnetic head along a predetermined radial direction for the magnetic disk medium of the FD inserted in the FDD. More specifically, the low density exclusive type FDD uses a stepping motor as the driving arrangement, while the high density type FDD uses a linear motor such as a voice coil motor (which may be abbreviated to "VCM") as the driving arrangement.
Description will now be made as regards the voice coil motor used as the driving arrangement of the high density type FDD. The voice coil motor comprises a voice coil and a magnetic circuit. The voice coil is disposed on the carriage at a rear side and is wound around a driving axis in parallel to the predetermined radial direction. The magnetic circuit generates a magnetic field which intersects current flowing in the voice coil. With this structure, by causing the current to flow in the voice coil in a direction where the magnetic field generated by the magnetic circuit intersects, a driving force occurs along a direction extending to the driving axis on the basis of interaction of the current with the magnetic field. The driving force causes the voice coil motor to move the carriage along the predetermined radial direction.
As well known in the art, the FD includes a magnetic disk medium accessed by a magnetic head. The magnetic disk medium has a plurality of tracks. The tracks include an outer most circumference track (which is named "Tr00") and an inner most circumference track. The outer most circumference track Tr00 is herein called the end most track. The small capacity FD has eighty tracks at one side.
It is necessary to position the magnetic head at a desired track position in a case where the FD is accessed by the magnetic head in the FDD. For this purpose, the carriage for supporting the magnetic head must be positioned.
In the low density exclusive type FDD using the stepping motor as the driving arrangement, it is possible to easily carry out the positioning of the carriage. This is because the stepping motor makes the carriage move step by step in response to pulses applied thereto in the low density exclusive type FDD. Accordingly, it is unnecessary for the low density exclusive type FDD to be provided with any exclusive positioning arrangement apart from the stepping motor.
On the other hand, in the high density type FDD using the linear motor as the driving arrangement, it is necessary to be provided with the exclusive positioning arrangement for positioning the carriage. This is because the linear motor makes the carriage freely move along the predetermined radial direction on the basis of current flowing in the coil and it is therefore necessary to carry out control to stop the free movement of the carriage. In order to position the carriage, it must be provided with a position detection unit for detecting a current position of the carriage or the magnetic head and a control unit for controlling the position of the carriage on the basis of the current position detected by the position detection unit.
A conventional position detection unit comprises a reflection scale mounted on a main surface of a main frame, a light-sensitive detector and a reflecting mirror which are mounted on the carriage in the manner which will later be described in detail in conjunction with FIG. 1. The light-sensitive detector includes a laser diode. However, with this structure, it is necessary to adjust a light axis for both of the light sensor and the reflecting mirror. In addition, the conventional position detection unit is expensive because the light-sensitive detector includes the laser diode. The conventional position detection unit requires a lot of parts because it comprises the reflection scale, the light-sensitive detector, and the reflecting mirror. Furthermore, the conventional position detection unit is disadvantageous in that the carriage has a bad tracking performance. This is because the carriage must be provided with the light-sensitive detector having a relatively heavy weight and the carriage is therefore too heavy.